The present invention relates to a coining method for treating the bonding pad surface of a lead frame to which wire bonding is applied.
When electronic parts such as transistors, diodes and ICs are assembled, lead frames are generally used for providing leads as external terminals. For example, FIG. 5 illustrates a typical construction of lead frames in the case of a transistor. The lead frames 1 are usually produced by punching out of a metal sheet and those formed into a desired shape are electrically connected to electrodes provided on the surface of a semiconductor element 2. In this case, the electrode of the semiconductor element 2 and the bonding pad surface 1a of the lead frame 1 are normally connected by wire bonding using a wire 3 of aluminum, gold, copper or the like.
As the surface of such a lead frame 1 is coarse, the contact face (wire bonding face 3a) of the wire 3 may adhere incompletely or insecurely to the bonding pad surface 1a if the wire bonding is applied to the surface as it is. In other words, the contact face tends to become unstable, thus causing conduction failure or causing the wire bonding face to peel off. In order to ensure the wire bonding, coining has heretofore been employed, as shown in FIG. 6 and FIG. 7 (sectional view taken on line D--D of FIG. 6), so that before the wire bonding, a coining surface 14 is formed by squashing and smoothing (coining) the whole bonding pad surface 1a at a depth of, for example, x=20 .mu.m.
In a case where the lead frame 1 is formed into a desired shape by punching, however, there develops a curvature 9 on the punched face 1b as shown in FIG. 8. For lead frames made of similar material, the thicker the lead frame 1 becomes, the greater,the curvature 9 becomes. Moreover, a burr 5 is produced over the substantially whole peripheral edge of the lead frame 1 on the opposite side from the punched face 1b above. When, for example, the thickness of the lead frame 1 is 700 .mu.m, the length y of the barr 5 amounts to about 30 .mu.m.
When an attempt is therefore made to do the coining by pressing down the lead frame 1 held between a cradle 6 installed beneath the back surface (punched face 1b) of the bonding pad surface 1a and a vertically-movable coining punch 7 with the side that has developed the burr 5 as the bonding pad surface 1a as shown in FIG. 9, the burr 5 projecting from the substantially whole peripheral edge of the bonding pad surface 1a acts as a buffer for lessening the effect of the pressure applied by the coining punch 7 between a punching face 7a and the bonding pad surface 1a. Thus, if the burr 5 is bulky, the coining depth x may not become uniform or otherwise pieces of the squashed burr 5 remain between the punching face 7a and the bonding pad surface 1a in such a way as to surround the central part of the bonding pad surface 1a, thus causing the force of repulsion to act uniformly against the pressure applied by the punching face 7a. As a result, the punching face 7a may be unable to effect the coining properly on the bonding pad surface 1a, particularly in the central part thereof or unable to effect the coining entirely because a space is formed between the punching face 7a and the bonding pad surface 1a (coining surface 14). Therefore, the reliable formation of the smooth coining surface 14 has been difficult.
Even though an attempt is made, on the other hand, to use the punched face 1b of the lead frame 1 as the bonding pad surface 1a as shown in FIG. 10, the burr 5 also acts as a buffer for lessening the pressure applied by the punch 7 between the cradle 6 and the punch 7. Moreover, the curvature 9 of the punched face 1b may allow the application of coining to only a part of the bonding pad surface 1a and there is the possibility that the wire cannot be connected properly thereto in case the wire-bonding position shifts. That is, even if the punched face 1b is used as the bonding pad surface 1a like this, it is still difficult to ensure the sufficient coining.
If the coining pressure is increased more than necessary so as to ensure the sufficient coining in view of the foregoing problems, the pressure will be applied excessively to the boundary of the coining surface 14, which also causes problems to arise newly in that the strength of the lead frame 1 at the boundrary may decrease or the lead frame itself may snap.